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Old 25th March 2004, 01:31 PM   #231
CH2 is offline CH2
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Quote:
Originally posted by jwb
For example:

IRFP044,140: Rθjc: .83, max Tj: 175°C, max Pd @ Tc=70°C: 126W
IRFP240,244: Rθjc: .83, max Tj: 150°C, max Pd @ Tc=70°C: 96W
IRFP044N: Rθjc: 1.3, max Tj: 150°C, max Pd @ Tc=70°C: 85W

But if power dissipation is your prime design concern, there is always the unholy IRFP260N:

IRFP260N: Rθjc: .50, max Tj: 175°C, max Pd @ Tc=70°C: 210W

Yeehaw. And a nice 5000pF input capacitance, also
I intend to use the 044N for a tiny AX. Since the power rating for 044N is only 85w, what is the comfort zone for this Mosfet? I was thinking of say 37.5w, am I living too dangerously?

Cheers
Hock Hua
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Old 25th March 2004, 01:52 PM   #232
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I was basing my guesstimations as to sound quality on the capacitance alone. I had neglected the transconductance, although it seems that it simply urges the sound along in the same direction. One of these days, I'll do a SOZ or something and sit down for a comparison listen.
Choice of devices is an arcane art. Assuming that you want to stick to IRF devices instead of Toshiba et. al., there are only two TO-247 P-channel parts: the IRFP9140 and the IRFP9240. So if you want to go complementary, you've got a limited playing field. If you're content to use N devices, you've got a whole basket-full to choose from. Then you get into capacitance, transconductance, and so forth. The thermal characteristics are important, of course, especially for a class A amp where you're going to be flogging them within an inch of their lives, but at least there you have the option of choosing to parallel devices that have other characteristics that you want. Of course, the capacitance adds up when you do that, but then...
Cascoding comes in the back door and negates some of the capacitance, in addition to decreasing some of the heat load. This is something that I've been playing with, but with different intentions from what Nelson has planned for the Zen series. I've got a one-dead-channel Hafler DH-200 (all right, let's have a show of hands...how many of you have one of those sitting around?) and it's beginning to annoy me. I'm hip deep in scrawled topologies, some of which have survived the "think about it while driving to work" test. The problem--besides money and time, the usual demons--is that I've got about five or six projects queued up ahead of that one...but it's the Hafler's heat sinks that bite my ankles as I walk by.
So many devices, so little time.

Grey

P.S.: I'd even considered using a tube front end for the Hafler rebuild, which would multiply the device choices further. Decided not to go that route, as hybrid circuits generally combine the worst of their characteristics, not the best. Also I wanted to keep it compact, and the high voltage supply was going to be hard to fit inside the chassis.
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Old 28th March 2004, 03:24 PM   #233
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Well still no avail w/ my ax... i've been working on a volume preamp using a TL08 (Crappy op amp...), but this project is definately not as important asg etting the amp to work...

one of my buddys in my town said that he 'didnt think enough voltage was getting to the fets', but he hasnt trouble shooted it, just talked about it...

Any ideas on where to start?...

Thanks alot, and thanks chad for the email
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Old 28th March 2004, 04:56 PM   #234
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BDP, have you recieved my email response? i've sent a couple, so something looks ike its screwy....


thanks
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Old 28th March 2004, 06:40 PM   #235
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KoolScooby,
Since no one else here sems to be of any help to you for what ever reason I'd be more than happy to troubleshoot it for you if you sent me just the board. I can use my own supply as its still in breadboard stage. Let me know if this would work out for you.

Or.... If you can supply me with a complete set of G, S ,D voltage readings on all the devices fomr one channel like I posted in a post previous to this I may also be able to help this way. I'd also like to know what type of tranny you are using and the VA rating of it. I wonder if your rails are collapsing too far for the amp to bias on and operate properly.


Grey,
You can already buy a Hafler with a tubed front end... just go visit Norm at Smart devices. He's a big Hafler fan.
Mark
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Old 28th March 2004, 07:39 PM   #236
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Quote:
Originally posted by Mark A. Gulbrandsen
KoolScooby,
Since no one else here sems to be of any help to you for what ever reason I'd be more than happy to troubleshoot it for you if you sent me just the board. I can use my own supply as its still in breadboard stage. Let me know if this would work out for you.

Or.... If you can supply me with a complete set of G, S ,D voltage readings on all the devices fomr one channel like I posted in a post previous to this I may also be able to help this way. I'd also like to know what type of tranny you are using and the VA rating of it. I wonder if your rails are collapsing too far for the amp to bias on and operate properly.


Grey,
You can already buy a Hafler with a tubed front end... just go visit Norm at Smart devices. He's a big Hafler fan.
Mark

Mark, Thanks very much for the offer ... that would be great, but i want to learn these troubleshooting steps on my own...

BDP told me he thinks my problem is my PS, and i believe it, because i'm just using a Variable DC PSU taht goes 0 to 25 and 1amp, so i'm pretty sure thats not adequate... he suggested me to work on a PS then the AX...

I'll try and get you a Set of GDS readings from my fets like you had earlier maybe tonight, if not tonight, then it might be as late as next thursday through next weekend, i'm going to a conference tomorrow thru wednesday... Thanks much mark and everyone else
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Old 28th March 2004, 07:59 PM   #237
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Actually its not necessary to get the readings yet. That PSU is definately way inadaquate to run this amp as the rails run around 3 amps each if you set the bias for .5 volts across the source resistors. I would highly reccomend what I'm using when I breadboard stuff and that is a 680 VA toroid powered through a 20 amp variac. I've got my primaries in series on the toroid so its 220 volts and then the secondary is about right when the variac is at 95% of full AC voltage. If I hook the primaries as 120 volts than the variac is about midway and the primary current is about 6.8 amps for a stereo pair!, when wired for 220 volts the AC current is half that.

Mark
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Old 28th March 2004, 08:06 PM   #238
BDP is offline BDP  United States
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Koolscooby,
Quote:
BDP, have you received my email response?
Yes, I replied via e-mail. Hope it helps get you going.
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Old 28th March 2004, 08:12 PM   #239
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...i mentioned the Power Supply in post #198...

i was trying my hardest to help!

Mark is on the right track though... we need some good readings
at certain points... the first being verifying a good PSU. It may be
that your test PSU just isn't up to the task.

maybe if this had a thread of it's own it would get more looks...
but extracting all the posts may be a bit of a pain.

but it looks like you're getting competant help now!
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Old 28th March 2004, 08:22 PM   #240
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Quote:
Originally posted by Mark A. Gulbrandsen
Actually its not necessary to get the readings yet. That PSU is definately way inadaquate to run this amp as the rails run around 3 amps each if you set the bias for .5 volts across the source resistors. I would highly reccomend what I'm using when I breadboard stuff and that is a 680 VA toroid powered through a 20 amp variac. I've got my primaries in series on the toroid so its 220 volts and then the secondary is about right when the variac is at 95% of full AC voltage. If I hook the primaries as 120 volts than the variac is about midway and the primary current is about 6.8 amps for a stereo pair!, when wired for 220 volts the AC current is half that.

Mark

Indeed.. my PSU is definately not up to par... BDP has emailed me suggesting some specs as well... how much did your 680VA Toroid go for ? and the 20 amp variac?

thanks much everyone
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